Add support for syncing untracked files

Allows you to specify untracked files which should be uploaded when
paired tracked files have been changed. File pairs are specified in
a new .git-ftp-include file using a syntax specified in the readme.

This is useful for uploading compiled files you wouldn't want to keep
under version control, like CoffeeScript -> JavaScript or SCSS -> CSS.
Add support for deleting untracked files
Quote paths and add -F to grep
Remove use of find, limit to 1-to-1 file linking
Update readme for explicit filename change
Fix broken untracked file deletion